1. Field of the Invention
The present invention relates to cooling modules such as those incorporating brushless DC fans or thermoelectric cooling devices, and particularly relates to cooling modules of a type suitable for use in personal computers and similar electronic equipment.
2. Description of the Related Art
Brushless DC fans have found wide use in cooling electronic equipment such as personal computers. Such fans incorporate a brushless DC motor (BLDC motor) which includes a stationary armature. At least two winding stages are configured around an armature core to provide a predetermined number of poles, and permanent magnets are mounted on the rotor of the fan. Frequently such fans incorporate two winding stages which are arranged to generate four magnetic poles in the armature, although greater numbers are possible. A traditional two-terminal BLDC fan motor usually incorporates circuitry to electronically commutate the winding stages to operate the motor at its full speed for the particular power supply voltage applied. Such circuitry frequently incorporates a rotor sensor to determine when the rotor has rotated sufficiently for the next winding stage to be energized. BLDC motors are also frequently known as electronically commutated motors.
BLDC fans have significant advantages over other kinds of fans. There is no rotating commutator or brush assembly as with traditional DC motors. Consequently, much fewer dust particles are generated which, if shed, may contaminate the equipment. There are fewer parts to wear out, and there is much less ignition noise than with a brush assembly. Furthermore, the magnetic coils of a BLDC motor are mounted on a rigid frame which improves the thermal dissipation of the motor and provides for greater structural integrity of the motor. BLDC fan motors are also much more electrically quiet than other kinds of motors previously used.
Despite all of these advantages, BLDC motors typically still wear out faster than the electronic systems they are included to protect. Mainly, such wear-out is caused by a failure of the bearings supporting the rotor assembly. Also, any fan generates acoustic noise which may be ergonomically displeasing to an operator of equipment incorporating such a fan. Likewise, as with all fans, the fan may become blocked by foreign matter, or filters included in the air flow path generated by the fan may become increasingly clogged with particulate matter which results in a higher strain on the motor driving the fan, and ultimately, an inability of the fan to provide the air flow for which the system was designed.
To reduce the detrimental aspects of using BLDC fans, fan-speed management has proven useful to drive such fans only when needed and only at a speed sufficiently useful to cool the system within which the fan is incorporated. Such fan-speed management results in less acoustic noise, either because the fan is totally off at certain times or is throttled to a much lower speed for a typical operation than its maximum speed which is only driven in a worst-case cooling demand. By throttling back the power of a BLDC fan, significant power dissipation is also saved, which is particularly important in battery-powered systems.
There are a variety of ways that BLDC motors, and particularly BLDC fans, have been controlled in attempt to provide fan-speed management. In its simplest implementation, such fans may be turned fully on or fully off. In a system incorporating such an on/off technique, the fan is turned on typically when a measured temperature somewhere in the system exceeds a first threshold limit, and the fan is turned off when that same temperature is below a second threshold limit (usually lower than the first threshold limit to provide hysteresis). This technique is effective when the fan is unnecessary for most operating conditions and may, therefore, be left off except in the most unusual circumstances. Nonetheless, such control has its drawbacks. For example, when the fan is on, it usually runs at full speed, in which case the acoustic noise is at its worst. Furthermore, if the temperature of the system hovers near the threshold temperatures, a displeasing frequent cycling of the fan may occur as the temperature is driven below the second threshold and allowed to rise above the first threshold with frequent cycling of the power to the fan.
In many systems, a temperature proportional cooling method has been devised to variably increase the speed of a cooling fan as a function of the heat generation or the temperature of the system to be protected. One method of controlling the speed of a brushless DC motor is to regulate the voltage powering the motor to a lesser voltage than the nominal operating voltage that the motor is normally specified to require. Referring now to FIG. 1, a standard brushless BLDC motor 40 is shown having one power terminal tied to ground, and the other power terminal connected to a triple darlington transistor circuit (i.e., transistors 41, 42 and 43) to a 12-volt positive power supply voltage. A variable control voltage, indicated in the figure as a variable voltage 44, is applied to the control terminal of transistor 41 and the current is amplified by transistor 42, and yet again, by transistor 43, to provide on the power terminal 45 of the BLDC motor 40 a substantially fixed voltage which is a function of the control voltage 44. Raising the control voltage 44 similarly raises the voltage applied to terminal 45, which is usually about 1.5 volts below the control voltage 44. The current drawn through BLDC motor 40 is generally proportional to the voltage applied across the motor. For example, if the current drawn by BLDC motor 40 at 12 volts is 300 milliamps, the current drawn through the BLDC motor 40 will usually be approximately 150 milliamps if terminal 45 is instead controlled to a 6-volt level. The linear voltage regulator circuit shown in FIG. 1 has several advantages, not the least of which it is extremely easy to design. It provides for a fan speed decrease which is almost linear with decreasing voltage. However, the power dissipation in the voltage regulator itself is substantial. At low fan speeds, the power dissipation of the regulator circuit may actually exceed the power consumed by the fan. This suggests large transistors may be necessary, and external heat sinks may need to be provided to dissipate the heat generated by such a large amount of current flowing through a circuit with a large voltage drop across the same circuit. Generating all of this additional heat seems counter-productive to the purpose of having the fan incorporated within the system in the first place. Consequently, such a linear voltage regulator fan-speed control has found decreasing use in recent years. Nonetheless, such a circuit does extend fan life if the system is designed to rarely require the fan to operate at full speed. Likewise, controlling the fan in this fashion may reduce acoustic noise typically generated by the system.
Another method of controlling the speed of a brushless DC motor involves chopping the power to intermittently apply full power to the motor at certain times, and applying no power to the motor at other times. This is usually accomplished with a relatively low-frequency (30-250 Hz) pulse-width-modulated (PWM) signal. One terminal of the BLDC motor may be connected to one power supply, and the other terminal may be connected to ground through a power switching device which is driven appropriately by the PWM signal.
Referring now to FIG. 2A, the brushless DC motor 40 discussed previously is here shown with one terminal connected to a +12 volt source of voltage and a second terminal 54 coupled through an MOS transistor 50 to ground. The gate of transistor 50 is generated by a pulse-width-modulated signal generator 51, which is responding to a control signal for controlling the speed of the BLDC motor 40. By using the pulse-width-modulated signal coupled to the gate of driver transistor 50, and assuming that transistor 50 is sized appropriately large, the average power consumed by transistor 50 is much lower than in the linear voltage regulator circuit described in FIG. 1. This occurs because driver transistor 50 is either fully turned on or fully turned off. When it is turned on by the pulse-width-modulated signal coupled to its gate terminal, the drain to source voltage is quite low (e.g., 0.5 volts) because the fan motor terminal 54 is pulled nearly to ground. While there may be significant amount of current flowing through driver transistor 50, there is not a large voltage drop across driver transistor 50, and so the instantaneous power during this portion of a cycle is very low. When driver transistor 50 is off, of course, no current flows, and there is no power dissipated instantaneously by driver transistor 50. Such a pulse-width-modulated speed control of a brushless DC motor, such as for a fan, has found wide use because of the much lower power dissipation compared to linear voltage regulator schemes, and due to the fact that a pulse-width-modulated signal at a 30 to 250 Hz rate is easily generated in either hardware or software and at very low cost.
Referring now to FIG. 2B, a similar circuit is shown incorporating a bipolar driver transistor 52, having a current limiting base resistor 53 connecting the output of the pulse-width-modulated signal generator 51 to the base terminal of transistor 52 to set the drive current into the base terminal and, consequently, provide for sufficient collector to emitter current through driver transistor 52 to fully support the current flow necessary through brushless DC motor 40.
A great many commercially available integrated circuits have been developed to generate useful pulse-width-modulated signals for controlling, in configurations similar to those shown in FIG. 2A and FIG. 2B, the speed of a brushless DC motor. Some of these circuits include temperature sensors which may be configured in a closed loop fashion to automatically control the speed of such a connected fan as a function of the locally measured temperature. Others of these integrated circuits include provisions for communicating with the whole system. Some generate the PWM signal to drive (either directly or through a buffering circuit) one of the power terminals of a standard two-terminal BLDC fan. Others generate a simple on/off control which may be used to control current through a BLDC fan.
Some types of brushless DC fans include a brushless DC motor which provides a tachometer output terminal. A pulse signal is generated on the tachometer terminal by the fan as a function of the internal self-commutation of the winding stages occurring internal to the fan. By sensing the various pulses generated on the tachometer output, the rotational speed of the fan may be determined, and an RPM (revolutions per minute) indication may be computed, or a fault condition, such as a stuck or locked rotor, may also be detected. Other commercial devices have been developed which provide a similar tachometer output or tachometer capability when using standard two-terminal brushless DC motors without a built-in tachometer output. Most such circuits generate the tachometer indication by including a low-valued resistor in the current path powering the fan motor. A voltage pulse across the resistor is generated when the commutation internal to the fan occurs because of the brief interruption of current flow through the winding stages within the fan motor.
Many commercial devices, particularly those incorporating temperature sensing capability, include provisions for sensing the speed of a fan, but lack any ability to control the speed of a fan. These are mostly used to sense alarm conditions, such as a clogged filter or an actually stuck rotor. One device, available from National Semiconductor Corporation, is known as the LM75. This device includes a serial interface compatible with the I2C(trademark) interface standard (initially developed by, and a trademark of Philips Corporation) for communicating with a host system, and includes a temperature sensor and an on/off output for a fan control. The I2C interface, which is also similar to, largely compatible with, and known by many as the SMBus interface (popularized by Intel Corporation, Santa Clara, Calif., and widely supported by many other companies), is a popular bi-directional two-wire serial interface, originally developed to provide power management control in battery-powered systems.
Other devices incorporate such an I2C interface and include a digital-to-analog converter (DAC) for generating on an output pin an analog voltage which can be used to control the speed of a brushless DC fan. For example, the THMC50, available from Texas Instruments, Inc., can be used to provide a controllable analog voltage on an output pin, which voltage may be used either in a linear voltage regulator, as described in relation to FIG. 1, or to modulate the pulse width of a PWM signal for controlling fan speed, as described in relation to FIG. 2.
One industry specification known as NLX calls for a zero to 10.5-volt analog signal which is used to set the fan speed. Controller integrated circuits are available to take this variable voltage signal and generate the pulse-width-modulated signal necessary to drive the power terminal of a fan motor. Such an analog control voltage may also be provided by a DAC output of a microcontroller or other device which may be incorporated within the system. And yet another standard, known as the EISCA standard, provides for the sensing of a variety of system voltages and system temperatures measured at several locations within the system, provides for a fan speed indication, and includes an I2C interface to communicate with the host processor. A great many industry-available devices provide for local and remote temperature sensing with an I2C bus interface. In another device available from Tracewell Systems, a Chassis Management Module product includes an I2C interface for communication with the host processor and a PWM channel output for each of two fans, for controlling each fan in a fashion as described in FIG. 2A and FIG. 2B. Yet another device, the LM78, available from National Semiconductor Corp., monitors temperature, voltage and fan speed, and includes an I2C interface.
In spite of these seemingly highly-specialized commercial solutions for various temperature voltage sensing and fan speed sensing and/or control, a variety of problems remain. Most notably, the internal commutation of the winding stages within the BLDC motor is asynchronous to the external power modulation (i.e., the xe2x80x9cpower choppingxe2x80x9d) of the fan power by the PWM signal driving the motor. The electrical noise is consequently worse. Also, the tachometer output may only be generated when the motor is energized by the PWM signal. This makes RPM determinations more difficult, particularly when the fan speed is much slower than full rate. Worse, the tachometer signal may have spurious noise spikes associated with the beginning and the end of the power switching due to the PWM signal applied to the power terminals of the fan motor. This may result in erroneous determination of the actual fan rotational speed and RPM. Moreover, all of these solutions require additional integrated circuits and other discrete components in addition to the fan itself to provide the cooling solution sought to be achieved.
Consequently, there is still a need for a better solution where thermal management, power control, sound level, and reliability are paramount.
An improved fan module includes integral fan control circuitry which independently implements a self-contained start and run motor control loop and also includes a communications port to accept digital commands from a host processor and to optionally provide status and other data in response to queries from the host processor. In one embodiment, the fan module interfaces with an internal system management bus, such as the SMbus or I2C bus, and accepts commands and provides status and data in a serial digital format. In other embodiments, a fan module interfaces with a parallel bus. A variety of commands such as on/off, and speed control settings may be received from the host system. The actual speed of the fan may be reported back when queried by the host system. In some embodiments the temperature of the fan or of the air flow through or in the vicinity of the fan may be reported back when queried. In still other embodiments, the current through one or more fan motor windings may be measured and reported back when queried.
Such an improved fan module is highly useful for personal computers, workstations, servers, embedded systems, and other electronic systems incorporating fans for cooling. The entire fan control circuit may be incorporated within or attached to the fan housing and a simple serial two-wire interface may be provided in addition to the two power terminals for the fan, resulting in a compact four-wire interface for the fan module. In one embodiment of the invention, the power coupled to the BLDC motor is not regulated to lower voltages, nor is it chopped in a PWM fashion, but is maintained at the fixed power supply voltage provided to the fan module. The power supply voltage may be chosen from, for example, 5 volts to 30 volts. All speed control, as well as on/off control, is communicated to the fan via the serial interface, and the speed is controlled independently of the particular power supply voltage chosen.
A fan module in accordance with the present invention affords all of the benefits previously achieved by varying the speed of brushless DC fans, such as enhanced ergonomics and enhanced reliability due to operating the fan speed, at least for a significant percentage of the time, at less than its full rated speed. In addition, however, system reliability is further enhanced by eliminating the requirement of additional integrated circuits and discrete components otherwise necessary to achieve the previously discussed forms of speed control. Moreover, by directly converting a digital speed command to appropriate signals driving the internal winding stages of the BLDC motor within the fan module, much less electrical noise is generated compared to that generated by chopping the power to a standard brushless DC fan motor. The digital command may include, for example, an eight-bit control word to indicate the desired speed, and consequently a much greater degree of control over the fan speed may be achieved than with other methods. Alternatively, a fan module may be configured to respond to only a few different speed commands.
Provision for responding to a variety of different commands may be designed into such a fan module and other capabilities included within its design. For example, the fan module may be designed to initially provide a default speed when first powered up (e.g., full-rated speed) to ensure adequate cooling without requiring any digital commands communicated to the fan module. After reaching a run state at the default speed, the fan module may then be commanded to a different speed as required by the system. Other queries which may be incorporated within such a fan module include responding to a manufacture and model number request, responding to actual angular velocity or RPM of the fan, providing the delta between the commanded speed and the actual speed, winding currents, and others. This capability allows a host system to determine when the fan is unable to achieve the speed commanded, indicating a possible clogged filter or high bearing wear, and would further allow such a system to alert a maintenance provider to schedule periodic maintenance of the fan before an outright failure of the fan. The fan module may also be designed to respond to a command to operate in an automatic mode which increases fan speed appropriately as ambient airflow temperature or other self-determined temperature increases without requiring supervision by the host system. In some embodiments, the fan is configured to control the fan speed as a function of a temperature measured in the vicinity of the fan, and no interface with a host processor is required. In still other embodiments, additional memory locations may be included in the fan control circuitry to allow storing of operational or environmental parameters of a fan module. The memory may be implemented as non-volatile memory to permit retention of such stored data even when power is removed. A wide variety of data may be measured and stored, such as peak and/or average winding current, cumulative rotations of the fan motor, current and/or peak temperatures within or nearby the fan module, cumulative operating time of the fan, cumulative time operated at each possible operating speed, and many others.
The teachings of the present invention may be utilized to achieve a variety of different embodiments. For example, a cooling device in accordance with the present invention may incorporate a thermoelectric cooler rather than a fan. In another embodiment, a clock signal may be used to convey a desired speed to a fan module, which desired speed is proportional to the frequency of the clock signal. Several such fan modules may be operated at precisely the same speed, thus eliminating distracting audible xe2x80x9cbeatxe2x80x9d frequencies otherwise present when nearby fan modules operate at similar but not identical speeds. The fan control circuits may or may not be incorporated as a part of the fan module. If separate from the module, such circuits may include more than one set of outputs for controlling more than one fan motor or other cooling device. Moreover, the control circuits may be incorporated to control a brushless DC motor used for other than a fan. Such control circuits may also be incorporated into a molded connector having one or more sets of motor control output connectors or wires, or may be implemented on a separate printed wiring board.